Label-free fiber-optic interferometric immunosensors based on waist-enlarged fusion taper

Label-free, all-fiber immunosensors based separately on Mach–Zehnder interferometry (MZI) and Michelson interferometery (MI) implemented using only single mode fiber (SMF) are proposed and experimentally demonstrated. The proposed SMF–MZI and SMF–MI sensors rely on intermodal interference between the core mode and cladding modes to detect ambient refractive index changes due to protein absorption on the cladding–ambient interface. The fiber surfaces were functionalized with self-assembled polyelectrolyte layers (chitosan (CS)/polysodium styrene sulfonate (PSS)). Immunoglobulin G (IgG) was immobilized on the polyelectrolyte layer and anti-immunoglobulin G (anti-IgG) molecular binding events were monitored through measurement of wavelength shifts. The proposed immunosensors exhibit anti-IgG detection sensitivities of 27.37 nm/(ng/mm2) and 5.91 nm/(ng/mm2) with concentration detection limits of 0.181 nM and 4.941 nM for MZI and MI sensor respectively. The specificities of the sensors were investigated using correlated/non-correlated anti-IgG–IgG pairs. These results demonstrate the feasibility of these sensors for various bio/chemical applications such as DNA hybridization detection and immunosensing.